1. Field of the Invention
The present invention relates to an antenna device. In particular, the present invention relates to an antenna device which makes it possible for a filter section to remove the noise current such as a surge current to the outside.
2. Description of the Related Art
In general, the high frequency radio equipment such as a portable telephone carries an antenna device which has an antenna for receiving a signal, and a filter unit which has a filter section for removing any unnecessary signals from the intended signal received by the antenna device.
In recent years, it is demanded to realize a light weight and a small size of high frequency radio equipment. In order to achieve this purpose, it is demanded to miniaturize the antenna device and the filter unit. In such a viewpoint, an antenna device has been suggested, which is provided with an antenna and a filter section (see, for example, Japanese Laid-Open Patent Publication No. 9-93015), because of the following reason. That is, such an antenna device is capable of receiving the intended signal and removing any unnecessary signals. Therefore, it is unnecessary to carry a filter unit on high frequency radio equipment. Further, it is unnecessary to perform a matching adjustment between the antenna device and the filter unit. Therefore, it is possible to improve the production efficiency of the high frequency radio equipment.
However, the antenna device as described above has no circuit for removing a noise current such as a surge current to the outside of the device. For this reason, for example, when the surge current flows through the inside of the antenna device as a result of the generation of static electricity in the vicinity of the antenna, the surge current flows to an electronic circuit such as an integrated circuit (IC) connected to the antenna device, because the surge current is not removed to the outside of the device. In such a case, the electronic circuit is sometimes damaged.
In order to avoid the flow of a surge current to the electronic circuit, it is preferable that a protective circuit is installed between the antenna device and the electronic circuit. However, in this case, it is necessary to provide a space for carrying the protective circuit on a mounting substrate of the high frequency radio equipment. Therefore, it is impossible to miniaturize the high frequency radio equipment described above. Further, when the protective circuit is carried, the weight of the high frequency radio equipment is inevitably increased. Consequently, it is impossible to realize a light weight of the high frequency radio equipment.
Although the inconvenience as described above exists, there has been no countermeasure for the noise current in an antenna device having an antenna and a filter section. That is, no proposal has been made until now to construct a circuit for removing the noise current.
The present invention has been made in order to solve the problems as described above, an object of which is to provide an antenna device which makes it possible to remove the noise current from a filter section to the outside of the antenna device, so that the electronic circuit is not damaged, and it is unnecessary to carry any protective circuit.
According to the present invention, there is provided an antenna device comprising a dielectric substrate; an antenna which is formed on an outer surface of the dielectric substrate; and a filter section which includes an input electrode, an input end side resonance electrode, an output end side resonance electrode, and an output electrode electrically connected to the antenna and which is formed at the inside of the dielectric substrate; wherein the antenna and the input end side resonance electrode form a short circuit.
Owing to the arrangement as described above, a noise current such as a surge current is removed to the outside of the device via a ground electrode which is short-circuited with the input end side resonance electrode.
In this arrangement, it is preferable that the output end side resonance electrode and the output electrode are coupled to one another via a capacitance, because of the following reason. That is, even when the noise current is returned to the filter section via the ground electrode and the output end side resonance electrode with one end short-circuited with the ground electrode, the noise current is suppressed by the capacitance formed between the output end side resonance electrode and the output electrode.
Therefore, it is possible to avoid the flow of the noise current through the electronic circuit connected to the antenna device. Accordingly, it is possible to avoid damage of the electronic circuit without installing a seperate member of a propective circuit.
It is also preferable that the input electrode and the input end side resonance electrode are electrically connected to one another; and the antenna and the input electrode are electrically connected to one another in the dielectric substrate via a through-hole.
It is also preferable that the antenna is formed to have a meander line configuration. It is also preferable that one or more resonance electrodes are formed between the input end side resonance electrode and the output end side resonance electrode.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.